A perfectly transparent film when placed between an observer and an object does not reduce the clarity or contrast of the object. The addition of color to a film may or may not have a visible affect the clarity or contrast of an object depending on the interactions that the chromophor has with light and the saturation. The transparency may be reduced by significant absorption of light or other interactions of light with a film. These interactions may also lead to an observed angle dependence for transparency.
The difficulty with most measurement methods for transparency is that they attempt to use a single value to define transparency, while several factors actually contribute to transparency. Complicating factors such as haze, scatter, gloss and bronze combine to affect transparency and influence the degree and angle dependence of the observed and measured transparency. When a single value is used to describe transparency little or no knowledge is obtained on which physical-optical properties are contributing to the reduction in transparency or their relative contributions. The present invention provides a measurement method of four major contributing factors that lead to a reduction of transparency utilizing a diffuse sphere spectrophotometer. The four factors are: Haze, Scatter, Gloss and Bronze.
Methods have been developed to measure transparency and haze, by both transmission through a film and reflectance off of a film.
Methods for measuring transmission transparency and haze are known. The American Society for Testing and Materials (ASTM) D-1003, and the ISO 13468 standard test methods, provide measures that are non-compensated and compensated for the sphere efficiency, respectively, for Total Transmittance for transparency and Transmission Haze. These methods use collimated light projected through the object into an intergrated sphere. In U.S. Pat. No. 6,294,638, method ASTM D-1003 is utilized to measure the haze and total light transmittance using a Datacolor SF600 Plus-CT for transparent thermoplastic polyurethanes. In U.S. Pat. No. 6,660,793 the Hunter ColorQuest is used to provide transmission haze and transparency. BYK Gardner also supplies haze-gard plus and haze-gard dual instruments for measuring transmission haze and total transmission.
Methods for measuring reflectance transparency and haze are also known. Systems have been developed to measure transparency and haze from one side of the material because films may be formed on opaque or translucent substrates or it may not be practical to position the test material between the light source and the detection system. H1,655 improves on U.S. Pat. Nos. 4,687,338 and 4,623,258 all to Task, et al by reducing directionality effects of the haze measurement by using a distributed annular light source for illuminating the transparent material. U.S. Pat. Nos. 5,451,253 and 6,706,863 teach the use of variation of the CIElab lightness, L*, to calculate the level of transparency. U.S. Pat. No. 6,706,863 teaches the conversion of the data to dL*, the difference between the L* for the conventional product and the samples, to show the degree of transparency. U.S. Pat. No. 5,451,253 teaches using of the dL* from a multi-angle spectrophotometer and measuring the difference in L* at 25° and 70° off of specular with an incident angle of 45° from normal. While both method provide a measure of transparency they do not provide a means for determining the source of the change in transparency.
ISO methods 2846 Part 1-4 describe methods for measuring transparency for printed inks using a 0/45 or 45/0 geometry spectrophotometer using the ΔEab color difference. The transparency measure is obtained by making multiple prints within a range of film thicknesses and measuring the ΔEab color difference relative to the black strip over which the ink was printed. The slope of the film thickness in microns plotted versus the ΔEab color difference provides the value for the transparency. The procedure states that negative slopes that are counterintuitive may be obtained. All of the methods that rely on a single value to define transparency have problems for consistently measuring highly transparent film with increasing or decreasing combinations of haze, scattering, gloss and bronze with varying film thickness or comparing a sample to a standard material. An example of this problem is a negative counterintuitive slope occurring in the ISO 2846 methods, that is easily explained using the present invention which separates the haze, scatter, gloss and bronze components of transparency.
Bronze effects have not been measured and have been misinterpreted. US (CIBA Bronze) describes a bronze pigment attributing the property to larger particle size. The bronze color observed in the patent is due to particle alignment not to particle size. Larger plate-like particles are more likely to align in the film providing the bronze effect evidenced by a bronze reflectance color observed for red pigments. It is the alignment of particles not the large particles that provide the bonze effect color. The bronze effect is readily apparent in a change in color that appears at the specular angle; for red pigments it appears as a bronzish color at specular, in blue and violet pigments it appears as a redish color at the specular angle. Bronze in a yellow film is described for the first time. Bronze in a yellow pigmented film provides a blue color at specular, but because blue & yellow are complementary colors this bronze is not visually observed as a color shift at specular but only as a higher gloss. Spectroscopically the bronze in a yellow film is readily observed when it is present.